Technical Presentations

Reservoir architecture of the Cretaceous Upper Burgan deltaic deposits in North Kuwait

Boris Kostic

The Cretaceous (Albian) Upper Burgan Formation has been on production for circa60 years in the Raudhatain and Sabiriyah fields in north Kuwait and significant resources remain. Key to an optimised development of this important reservoir is an improved geological understanding of its reservoir architecture and connectivity within a robust stratigraphic layering framework.

Information from all cores available to date and pore to field-scale analyses of the latest 17 cored wells assessed in detail in this study confirm that the Upper Burgan forms a depositionally complex reservoir characterised by a relatively high degree of spatial and temporal facies variations.Two end-member depositional models are proposed: a tidally-influenced, fluvial-dominated delta model and a more marine-influenced shoreface model that dominate specific stratigraphic layers in each field, respectively.The layering framework is based on the recognition of at least field to subregional-scale correlation surfaces, typically bounding distinct phases of shoreline progradation and/or retreat.The latter are stacked within an overall outbuilding followed by a gross backstepping delta system with the system turnaround occurring at different stratigraphic levels in the Raudhatain and Sabiriyah fields, clearly highlighting the lateral migration of the delta lobe system across the two fields with time.In this sense the Upper Burgan represents more proximal highstand deposits during times of increased deltaic sediment influx relative to the underlying Middle Burgan, which is dominated by marine sandstones and mudrocks.

This depositional organisation, supported by facies correlation and mapping facilitates the prediction of key reservoir sandbody distribution and in fact reservoir quality, as this is mainly depositionally controlled.For instance, despite the complex reservoir architecture and delta switching, the distribution and orientation of the main, circa10-50ft thick, multi-storey channel-fill sandbodies appears to be reasonably well predictable within relatively stable, km-wide zones in the various layers.These clean sandbodies tend to be fine to locally medium-grained and support a macropore-dominated pore system, which is why they represent the best reservoirs in the Upper Burgan in both fields, most notably below the turnaround of the delta system.The comparatively poorer reservoir quality in the associated mouthbar and sandflat deposits is a function of their slightly finer grain size and locally higher ductile content resulting in the development of a mixed macro to micropore-dominated pore system.These reservoir facies are interbedded with non-reservoir mudrocks that were deposited in a variety of marginal marine settings ranging from muddy tidal flats to more sheltered, shallow-water embayments/interdistributary bays.They are considered to act as background baffles and even permeability barriers between the major paralic sandbodies given their mud-prone composition.

The development of laterally extensive, marine-influenced shoreface sandsheets and offshore mudrocks during phases of diminished deltaic influx and/or comparatively high relative sea level is stratigraphically controlled, which means that the distribution of these poorer quality sandsheets and mudrock barriers is well constrained in both fields.In contrast, the mapping of thinner, circa3-15ft thick, dominantly single-storey channel-fill reservoir sandbodies remains a challenge without a dense well control, but unravelling their distribution could be a critical aspect for future field development.